Global Navigation Satellite Systems (GNSS) include the Global Positioning System (GPS), the GLONASS system, the proposed Galileo system, and the proposed Beidou system.
In traditional RTK (real-time kinematic) GNSS positioning, the rover receiver (rover) collects real-time GNSS signal data and receives reference data from a base station or a network of reference stations. The base station and reference stations receive the GNSS signals at the same instant as the rover. Because the reference data arrives at the rover with a finite delay (latency) due to processing and communication, the rover stores its locally-collected data and matches it epoch by epoch with the arriving reference data. The rover then computes a synchronous position for each epoch using the matched data for that epoch. The synchronous data processing of traditional RTK positioning uses only matched reference and rover data to obtain the maximum possible accuracy. The need to wait for the reference data means that the computed position is always delayed with respect to the current position of the rover as reflected in the current GNSS data from the rover's GNSS-signal tracking loops. While the synchronous position computed by the rover is accurate, the accuracy comes with an inherent delay.
A prior-art delta-phase method used in kinematic survey is aimed at producing low-latency estimates of the rover position without waiting for the matching (synchronous) reference data to be received. When synchronous reference data is available for a given epoch, the rover uses it to compute a synchronous position for that epoch. When synchronous reference data is not available for a current epoch, the rover estimates its delta position (the change in rover position) from the last synchronous epoch until the current epoch) and adds this delta position to the last synchronous position to obtain a current low-latency position estimate while awaiting reference data for a further synchronous epoch. The price of this low-latency scheme is an additional error of about 1 mm per second of time difference between rover and reference data.
To maximize accuracy, prior-art processing for static-point surveys uses only synchronous data for epochs when the rover is static at the survey point, and also for ambiguity estimation (receiver initialization). The delay due to waiting for synchronous data has not been an issue in the prior art because static occupations have traditionally been in the range of at least a few seconds and because ambiguity resolution takes at least a couple of seconds.
Improved methods and apparatus for processing GNSS signals are desired, particularly to improve productivity in static position surveying with GNSS receivers.